ZMAT2

ZMAT2 (hSNU23/Snu23) is a spliceosomal protein that contributes to the assembly and catalytic activation of the pre-catalytic B complex (PMID:28530653, PMID:39995036). It nucleates a conserved trimeric subcomplex with PRP38 and MFAP1/Spp381, contacting PRP38 through an ER/K-motif-stabilized single α-helix and acting as an intermittent scaffold that facilitates spliceosome remodeling (PMID:27773687, PMID:28335716). Within the B complex, ZMAT2 binds the PRP8 N-terminal domain and stabilizes the U6 ACAGAGA stem–pre-mRNA and Brr2–U4 snRNA interactions (PMID:28530653), and in the human complex it interacts with FBP21 and PRP38 at the U6/5′ splice site helix to support 5′ splice site recognition (PMID:38383864). Real-time single-molecule imaging established that ZMAT2 binds and releases from the spliceosome together with PRP38 and Spp381 as a single BCP module, associating after tri-snRNP binding and dissociating predominantly after U4 snRNP release upon NTC association (PMID:39995036). Beyond core splicing, ZMAT2 maintains epidermal keratinocytes in an undifferentiated, proliferative state by binding and correctly splicing cell-adhesion transcripts in functional cooperation with chromatin modifiers (PMID:30380419), undergoes phase separation into condensates that regulate alternative splicing of TRIM28 mRNA to control ROS levels and proliferation (PMID:39164737), and acts as a regulator of skeletal development through the BMP signaling pathway (PMID:32247068).

1. 2016 High

   Established the structural basis by which ZMAT2 engages its splicing partners, answering how the protein is organized within a remodeling module.

   Evidence Crystal structure, in vitro binding, and ER/K-motif mutagenesis of the Snu23–Prp38–MFAP1 trimer

   PMID:27773687

   Open questions at the time

   • Did not resolve how the trimer docks onto the full spliceosome
   • Functional consequence of disrupting the ER/K helix in cells not tested
2. 2017 High

   Placed ZMAT2 within the pre-catalytic B complex and defined its RNA-stabilizing role, answering what the protein physically does during spliceosome assembly.

   Evidence Near-atomic cryo-EM of the yeast B complex spliceosome

   PMID:28530653

   Open questions at the time

   • Static structure does not capture binding/release dynamics
   • Human complex architecture not directly resolved here
3. 2017 Medium

   Demonstrated evolutionary conservation of the Snu23–Prp38–MFAP1/Spp381 subcomplex across yeast and metazoa, answering whether the module is repositioned between species.

   Evidence Cross-species in vitro binding and pulldown experiments with ortholog inference

   PMID:28335716

   Open questions at the time

   • Binding assays in vitro, not in assembled spliceosomes
   • Single-lab data
4. 2018 Medium

   Identified a tissue-level role for ZMAT2 in maintaining epidermal progenitor identity through splicing of adhesion transcripts, extending its function beyond generic core splicing.

   Evidence siRNA knockdown, RIP, transcriptome splicing analysis, and protein-interaction validation in keratinocytes

   PMID:30380419

   Open questions at the time

   • Mechanistic link between adhesion-transcript splicing and chromatin modifiers not resolved
   • Direct vs indirect effects on individual transcripts unclear
5. 2018 Medium

   Linked ZMAT2/Snu23 to the DNA damage response as a phosphorylation target of Rad53, raising the possibility that splicing assembly is regulated by checkpoint signaling.

   Evidence Phosphoproteomic screen and in vitro kinase assay in yeast

   PMID:30377154

   Open questions at the time

   • Functional consequence of phosphorylation not established
   • In vitro phosphorylation only; physiological relevance untested
6. 2020 Medium

   Established ZMAT2 as a regulator of skeletal development via BMP signaling, answering whether the gene has an organismal developmental function.

   Evidence Zebrafish morpholino knockdown with wild-type and mutant mRNA rescue

   PMID:32247068

   Open questions at the time

   • Molecular link between ZMAT2 splicing activity and BMP pathway not defined
   • Morpholino approach without genetic mutant confirmation
7. 2024 High

   Resolved ZMAT2 contacts in the human B complex and connected them to 5′ splice site recognition, extending the yeast structural model to humans.

   Evidence Cryo-EM of human spliceosomal B complex dimers

   PMID:38383864

   Open questions at the time

   • Functional role of B-complex dimerization not established
   • Dynamics of FBP21–SNU23 contact not captured
8. 2024 Medium

   Revealed a non-canonical condensate-forming behavior of ZMAT2 that regulates alternative splicing of TRIM28 and controls ROS and proliferation in cancer cells.

   Evidence RNAseq, RIP-seq, condensate imaging, ROS and proliferation assays in HCC cells

   PMID:39164737

   Open questions at the time

   • Relationship between condensate formation and core spliceosome function unclear
   • Generality of phase separation beyond TRIM28 not assessed
9. 2025 High

   Defined the real-time assembly order of the BCP module, answering when ZMAT2 binds and is released relative to tri-snRNP and U4 dissociation.

   Evidence Single-molecule CoSMoS imaging of splicing dynamics in yeast

   PMID:39995036

   Open questions at the time

   • Whether human BCP follows identical kinetics not directly imaged
   • Regulation of the low-ATP pre-association state in vivo unknown

• How ZMAT2's core spliceosomal role mechanistically connects to its tissue-specific functions in epidermal identity, BMP-dependent skeletal development, and condensate-driven alternative splicing remains unresolved.
• No unified model linking core B-complex activity to selective target splicing
• Physiological role of phosphorylation and phase separation not integrated